1. Field of the Invention
The present invention relates generally to a piston structure for such a type of internal combustion engine. More particularly, the invention relates to a piston structure for such a type of internal combustion engine which has a lubrication system injecting lubricant into internal space of an engine cylinder via a lubricant injection nozzle.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 57-89812 discloses a lubrication system for a piston of an internal combustion engine. The disclosed lubrication system has a lubricant injection nozzle projecting into the internal space of an engine cylinder in order to supply lubricant into the internal space of the engine cylinder. The lubricant injection nozzle is placed at higher position to the orientation where the lower edge of a skirt portion of the piston is placed at bottom-dead-center (BDC).
In order to permit the piston to stroke to BDC, a cut-out is formed in the lower edge of the skirt of the piston for receiving the lubricant injection nozzle while the piston is in a position in the vicinity of BDC. Such a piston construction avoids the necessity of extra height of the engine cylinder for providing the lubricant injection nozzle and permits a reduction in the overall height of the engine cylinder block. However, on the other hand, such construction of piston with the cut-out inevitably forms thin portion around the cut-out. Such thin portion tends to cause concentration of stress resulting in formation of crack for damaging the piston.